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Pests and diseases



Though specialist books describe a discouraging number of pests and diseases that can attack a garden, only a few of them are likely to become a problem in any one year. Even then, with the exception of the seedling stage, the destruction they wreak is seldom complete – something edible can usually be salvaged from even the most desperate-looking crop. Nevertheless, it is essential for any gardener’s sanity to do everything possible to keep damage from pests to minimum so that all the hard work that goes into a garden is not wasted.

Knowing the enemy

Pests are herbivorous animals, and include mammals as large as deer and as small as moles and mice; birds such as pigeons; all manner of insects from larvae to adults; slugs and snails; mites; and microscopic nematodes. They suck, chew and burrow their way through seeds, seedlings and mature plants, and find their way into a garden by walking, crawling and flying.


Diseases are microscopic parasites that live off plants. Also known as pathogens, these organisms include bacteria, fungi and viruses. They are moved around on seeds and transplants, and some can live in the soil for years. Fungi and bacteria are blown around by the wind, while viruses are often transported by sap-sucking aphids.



Prevention and cure

Total control over pests and diseases is neither possible nor necessary in a home garden. The goal, instead, is damage limitation using techniques that maintain a balance between the needs of the gardener and the environment.

  • From the ground up

Producing fit and healthy plants is the first line of defence against pests and diseases. This can only be achieved if the site and soil are suitable for crop production, and any deficiencies in the soil must be corrected before growing begins.

Composts are magic elixirs, and their applications to soils can help suppress or control diseases. They must never, however, become a source of infection themselves, and plant material infected by persistent soil borne diseases such as allium white rot and brassica club root should never go into the compost heap.


Well-designed rotations help promote a healthy soil. To be most effective, they should be planned so that plants of the same family are grown on a different piece of ground each year.

  • Keep it clean

A clean garden is a healthy garden, and close attention to hygiene will pay dividends. Boots, tools and equipment can be carriers of soil-borne pests and diseases, and these must be thoroughly washed before they move from one garden to another. Crop debris and weeds should be also cleaned up, and only disease-free seed and planting material, including transplants and potato tubers, should be used to start a crop.

  • Good culture

Overwintered vegetables can act as a host to some pests and diseases that then spread onto spring-grown crops. Winter brassicas, for example, can be a source of aphids and whitefly, and they must be removed before the spring crops are planted.

Diseases can build up in stagnant air surrounding vegetables and herbs. Growing crops at wider-than-normal row spacing promotes better air movement through the canopy and helps keep disease levels down. Venting tunnels and greenhouses, even in winter, does much the same thing by renewing and refreshing the air inside.

 Ashbrook lettuce with a wide spacing

Wide spacing reduces stagnant air and keeps diseases down.

Sowing and harvest times can be adjusted to avoid pests and diseases. Carrots are attacked by early and late generations of carrot root fly, and delaying the sowing of main crop varieties until June misses the first generation. Outdoor tomatoes, too, are more likely to avoid blight damage if early maturing varieties are grown.

Resistant varieties are powerful allies in the fight against both pests and diseases. If carefully chosen, they can reduce the damage of blight in tomates, root fly in carrots, rust in leeks and mildew in lettuce.

  • Barriers

Physical barriers of various designs can be employed to keep pests away from vegetable crops. Home-cut circles or squares of carpet foam put at the base of brassica plants will keep out cabbage root fly. Likewise, a wall of plastic sheeting, mesh or fleece about 45cm high can be erected around the carrot patch to protect it against carrot root fly.

A versatile and easy-to-use barrier for both large and small areas are the fleeces and fine mesh nets reminiscent of mosquito netting. Laid over a crop, these are good all-rounders for detering deer, rabbits, pigeons and flying insects, and can even keep the dreaded flea beetle away from brassicas if put down as soon the crop is sown.

Fleece can be used to keep pests of growing vegetables Fleece is a very fine material to keep pests away from growing vegetables

Fleece can act as a barrier to keep deer, rabbits pigeons and flying insects off a vegetable crop.

  • Natural-born killers


Predators and parasites of pests live naturally in and around the garden. These ‘beneficials’ are carnivorous that quietly about their business of eating the pests and keeping their populations down. They include, for example, birds, hover flies, ladybirds and lacewings.

Aphids and aphid mummies on a chilli plant leaf

Aphids on the underside of chilli leaves can be control by parastic wasps that lay their eggs in the aphid, making them turn into "mummies".


The garden environment can be manipulated to encourage more of the beneficials to come into the garden. For example, putting up hospitable nesting boxes provides safe havens where birds can live and nurture their young. Growing flowering plants like phacelia and Michaelmas daisy also lures in the smaller beneficials like hover flies. Habitats can also be created to provide these silent allies with an overwintering refuge from which they can launch their terrorist attacks the following growing season. The habitats do not have to be complicated to set up, and can be simple, home-made affairs such as flower pots loosely packed with straw that provide shelter for ladybirds and lacewings.

Specialist companies offer an ever-increasing selection of natural enemies of pests that can be ordered through the post. These biological control agents include the pathogenic bacteria, Bacillus thuringiensis, that kills caterpillars in cabbages and kales; parasitic nematodes used against soil-borne slugs; and parasitic wasps released in tunnels and greenhouses to control aphids. Given the right environment, these control agents are effective in keeping pest populations down to an acceptable level, though they work best in the early stages of pest attack.

  • Mixing things up 

Interplanting or mixed planting is often promoted as a method of pest and disease control. Most of the recommended combinations fall into the realm of old wives tales, and are supported with little concrete evidence that justifies their use. Some, however, stand up to closer scrutiny and might actually work: French marigolds may keep whitefly out of greenhouses and tunnels; onions grown with carrots might reduce carrot root fly damage; and a mix of resistant and susceptible lettuce varieties seems to protect against downy mildew. Interplanting is worth trying, but only if it is approached with an open and curious mind that enjoys a trial-and-error approach to gardening.

  • Physical

Pests and diseases can be physically removed to prevent their spread in the garden. For example, pulling out infected plants slows the movement of disease to unaffected areas, while crushing aphids between the fingers or washing them off with a hose pipe works well when infestations are small. Caterpillars, slugs and snails can be picked off plants, and is a good job for neighbourhood children wanting to earn pocket money.


  • Biocides

Biocides are toxins used to control pests and fungal diseases. Known generically as pesticides and fungicides, they come in either liquid or powder form and are applied directly to growing plants. Though effective, these biocides are not necessarily the ideal solution to pest and disease problems. Pesticides, for example, are indiscriminate killers that can knock out the natural enemies of pests as well as the pests themselves. Likewise, both fungicides and pesticides, when used incorrectly (like on a windy day), can drift onto neighbouring gardens where they may not be wanted.

The use of biocides is controlled by law and subject to Pesticides Safety Directorate regulations. This means that traditional plant-based formulations made at home are illegal, and that the commercial products available from garden centres and specialist suppliers are subject to strict controls. Given the legal constraints and negative impact of biocides, their role in a vegetable garden should be seen for what it is: a desperate measure to be employed only after everything else has failed.


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